The purpose of this proposal is to clarify how ion transport properties of the respiratory epithelium change during development to convert the mammalian lung from a liquid-filled, Cl-secreting organ before birth to an air-filled, Na-absorbing organ after birth. The general strategy is to define developmental changes of ion transport in cells derived from 2 distinct regions of the respiratory tract epithelium (trachea, distal lung) of 2 species (rabbits, sheep), and to relate these changes to developmental differences in lung liquid balance before and after birth. The longterm objective is to improve understanding of basic mechanisms responsible for reversal of liquid flow across the lung epithelium near birth, which should provide useful information for developing effective measures to inhibit or promote resolution of pulmonary edema in infants, children and adults. Specific aims focus on the central hypothesis that events associated with labor and adaptation at birth, in particular the release of adrenal hormones (cortisol, aldosterone, and epinephrine), influence epithelial cell metabolism and Na-K-ATPase activity, with a resultant switch from Cl-secretion to Na-absorption across the bronchopulmonary epithelium. The project uses 4 complementary experimental approaches to study mechanisms and regulation of Na and Cl transport in the respiratory epithelium of developing animals: (1) airway and distal lung epithelial cells will be isolated from fetal, newborn and adult animals of 2 species in order to measure fluxes of radiolabeled ions (86Rb,36Cl, 22Na) in the presence or absence of various inhibitors of ion transport; (2) bioelectric and ion transport properties of confluent monolayers of cultured airway and distal lung epithelial cells will be assessed with and without prior exposure of the cells to specific hormones (glucocorticoids, aldosterone, beta-adrenergic agonists) that increase in concentration late in gestation; (3) net production (secretion and absorption) of luminal liquid will be measured in isolated, blood-perfused lungs of fetal and newborn animals (rabbits, sheep) under basal and experimental conditions (hormone exposure, treatment with stimulants and inhibitors of epithelial ion transport); and (4) net liquid production, ion concentrations and transepithelial electrical potential difference will be measured in lungs of fetal sheep before and during labor, with and without prior exposure to hormones, and in the presence or absence of various drugs that influence epithelial ion transport. These complementary experimental approaches permit (a) assessment of ion transport properties at different regions and in different cells of the developing respiratory epithelium; and (b) clarification of their respective contributions to liquid secretion and absorption under relevant physiological conditions in lungs of living animals. This work may have important implications concerning fluid balance in the newborn lung and breathing disorders associated with respiratory epithelial dysfunction.